1. Field of the Invention
The present invention is related to a smectic liquid crystal compound and a liquid crystal composition containing said liquid crystal compound, particularly to a smectic liquid crystal compound and a liquid crystal composition excellent in response and highly suitable as the material for ferroelectric liquid crystals.
2. Description of the Prior Art
Liquid crystal display is widely applied at present. Twisted nematic (TN) type display is applied in most of them, which is advantageous because of low electric power consumption and little fatigue of eyes, as it is of light-receiving type. On the other hand, however, it has a disadvantage of long response time. High-speed response has been required recently for display devices, and liquid crystal materials are being improved for this purpose. However, there is still a great gap in response time between this type of materials and those of luminescent type such as light emitting diode, EL (electroluminescence), or plasma display. Approaches have been continued therefore to find a novel display method in which response can be made with high speed, with advantages of the light-receiving type of liquid crystal display consuming small electric power. One of these approaches is a display device in which an optical switching phenomenon of ferroelectric liquid crystals is utilized (see, e.g., N. A. Clark and S. T. Lagerwall in Appl. Phys. Lett. 36, 899 (1980)). A ferroelectric liquid crystal was discovered by R. B. Meyer et al. (J. Physique, 36, L-69 (1975)), and considered to belong to a chiral smectic C phase (hereinafter abbreviated as S.sub.C *) or to a chiral smectic H phase (hereinafter abbreviated as S.sub.H *) based on it molecular orientation. The compounds listed in Table 1 have been heretofore known as ferroelectric liquid crystal compounds (Ph. Martino Lagarde, J. Physique, 37, C3-129 (1976)).
In Table 1, C stands for a crystal phase, S.sub.C * for a chiral smectic C phase, S.sub.H * for a chiral smectic H phase, S.sub.A for a smectic A phase, and I for an isotropic phase.
TABLE 1 __________________________________________________________________________ No. Chemical structure Phase transition temperature (.degree.C.) (note) __________________________________________________________________________ ##STR3## ##STR4## 2 ##STR5## ##STR6## 3 ##STR7## ##STR8## 4 ##STR9## ##STR10## __________________________________________________________________________
These ferroelectric liquid crystal compounds have a C--C double bond (--CH.dbd.C&lt;), which results in weak optical resistance, that is, the compound is easily isomerized with light in a short period into cis-form, not displaying a liquid crystal phase. The compounds are not therefore suitable for practical use as liquid materials for display elements. The inventors have arrived at the present invention as a result of studies with the purpose of providing compounds with a stable molecular structure.